This invention relates to copolymer compositions comprising a major amount of an alternating copolymer of carbon monoxide with one or more olefinically unsaturated compounds and a minor amount of at least one stabilizing additive.
The relevant copolymers are linear and they are characterised by having a repeating structure [A--CO] which implies that a unit A is positioned left and right in between carbonyl units. A is a unit derived from an olefinically unsaturated compound. The term "copolymer" includes terpolymers in which different units A are present. Thus, the copolymers are copolymers of carbon monoxide and one or more olefinically unsaturated compounds. Examples of suitable olefinically unsaturated compounds yielding a unit A are ethene, propene, butene, octene, styrene and acrylate esters. The copolymers mentioned above are known and are disclosed in EP-A-121965 and EP-A-181014. Whilst these copolymers have attractive physical and mechanical properties such as yield stress, tensile strength, impact strength and flexural modulus, in some instances, their processing properties may leave room for improvement.
Copolymers with a high intrinsic viscosity, or limiting viscosity number (LVN), have better physical properties as engineering thermoplastic than copolymers with a lower intrinsic viscosity. Melt processing, e.g. extrusion, of copolymers, especially when they have an LVN of above about 2.0 dl/g (measured at 60.degree. C. in m-cresol), is adversely affected by a poor melt stability which is apparent from a notable increase of the viscosity of the melt with increasing residence time. In particular in fibre and sheet applications, melt-extrusion is a critical step, even for copolymers with a low LVN. It is evident that this problem of the instability of the copolymer melt needs to be solved.
Copolymer compositions comprising a major amount of an alternating copolymer of carbon monoxide with one or more olefinically unsaturated compounds and a minor amount of at least one stabilizing additive are known in the art. For example, the stabilizing additive may be an aluminium hydroxide, such as bayerite. Whilst the addition of such a material to the copolymers brings about an improvement of the stability of the melt, it appears that especially for melt processing operations in commercial production of moulded parts and extrusion at commercial scale of fibres and sheet, the melt stability could be improved even further. It is an object of the present invention to provide copolymer compositions having further improved melt stability.
It has now been found that a high degree of melt stability of the copolymers can be achieved by the addition thereto of a zeolite-type trivalent-metal silicate which contains a cation of an element of Group 2 of the Periodic Table. It is known that zeolites which contain sodium, potassium, calcium, zinc, etc. can be used in random polymers of ethene, vinyl acetate and carbon monoxide in which the amount of ketonic carbonyl groups is substantially lower than in the present copolymers, which are linear and alternating. Surprisingly, it has been found that in order to melt stabilise the linear alternating copolymers the zeolite-type trivalent-metal silicate should contain a cation of a Group 2 element, whilst zeolite-type trivalent-metal silicates which contain cations of other elements, such as sodium and potassium, appear to destabilise the copolymers.
In addition to the high degree of melt stability which can be achieved by the addition of a zeolite-type trivalent-metal silicate, melt processing of the copolymers in the presence of such a material has another unexpected benefit in that it reduces the amount of residual solvent, which may have been used in the synthesis of the copolymer and which may still be present in the copolymer, even after conventional purification. This is unexpected since the removal of residual solvent by physical means such as extrusion of the copolymer in the presence of water or steam and the application of vacuum during extrusion failed to reduce the amount of residual solvent. This finding is important when the polymer composition is to be used in the production of articles for certain packaging applications.